The instant invention relates to a catalyzed diesel soot filter and process for the reduction of the ignition temperature or oxidation temperature of Diesel soot filtered out of the exhaust gas of Diesel engines by a Diesel soot filter.
Diesel engines emit soot particles referred to herein as “Diesel soot”. Diesel soot filters have been developed; see, for example, U.S. Pat. No. 5,098,455 to Doty, Moyer and Hughes, to filter Diesel soot from Diesel engine exhaust gas. Many prior art Diesel soot filters employ a honeycomb filter element encased in a sheet metal shell. The honeycomb filter element has an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls. A fraction of the total number of cells at the inlet end are plugged and the remaining cells are plugged at the outlet end in a designed pattern so that Diesel engine exhaust flows through the porous walls of the filter element so that Diesel soot is filtered from the exhaust on and into the porous walls of the filter element. As discussed in U.S. Pat. No. 4,515,758 to Domesle et al., the temperature of Diesel engine exhaust gas under normal operating conditions is not sufficient to burn off the accumulated soot. Therefore, the back pressure of the exhaust gas through the filter increases over time to unacceptable levels.
One solution to the problem of soot accumulation in a Diesel soot filter is to periodically increase the temperature of the exhaust gas by, for example, electrically heating the exhaust gas or by burning an auxiliary fuel in the exhaust gas, to ignite and burn away the accumulated soot from the filter. Heating the exhaust gas to solve the problem of soot accumulation in the filter does, however, increase the fuel consumption of the Diesel engine.
A more efficient solution to the problem of soot accumulation in the filter is to coat the filter with a catalytic agent that reduces the temperature of ignition or oxidation of the accumulated soot. For example, the above referenced '758 patent coated a Diesel soot filter with a catalyst selected from lithium oxide, copper chloride, vanadium oxide/alkali metal oxide combinations, a vanadate of an alkali metal or of cerium, or a silver or alkali metal perrhenate, preferably of sodium or silver, or mixtures of these substances in order to reduce the ignition temperature or oxidation temperature of the accumulated soot. U.S. Pat. No. 6,248,689 to Manson disclosed a catalytic agent incorporating Platinum Group Metals (PGM's) that reduced the ignition temperature or oxidation temperature sufficiently so that the catalyzed diesel soot filter was self-regenerating at most diesel exhaust gas temperatures.
Despite the past advances made in the development of a Diesel soot filter catalyzed to reduce the ignition temperature or oxidation temperature of soot trapped therein, it would be a further advance in the art if such a catalyzed Diesel soot filter were discovered that used a less expensive catalyst.